Fix implicit fallthroughs throughout Skia.

This CL is not fully comprehensive; for instance, it does not contain
fixes for backends that don't compile on Mac. But it does resolve the
vast majority of cases that trigger -Wimplicit-fallthrough.

A few minor bugs were found and fixed, but none that were likely to
affect normal operation.

Change-Id: I43487602b0d56200ce8b42702e04f66390d82f60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295916
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/gpu/GrOpsTask.cpp b/src/gpu/GrOpsTask.cpp
index 1642832..ec2c6c9 100644
--- a/src/gpu/GrOpsTask.cpp
+++ b/src/gpu/GrOpsTask.cpp
@@ -556,6 +556,7 @@
             // once finished, meaning the stencil values will always remain cleared after the
             // initial clear. Just fall through to reloading the existing (cleared) stencil values
             // from memory.
+            [[fallthrough]];
         case StencilContent::kPreserved:
             SkASSERT(stencil);
             stencilLoadOp = GrLoadOp::kLoad;